These days, compact, slim imaging devices are incorporated in compact, slim electronic appliances called portable terminals (for example, cellular phones, PDAs (personal digital assistants), etc.). Between such a portable terminal and, for example, an electronic appliance at a remote location, information such as sound information and image information is transferred bidirectionally.
Image sensors used in imaging devices are, for example, solid-state image sensors such as CCD (charge-coupled device) image sensors and CMOS (complementary metal-oxide semiconductor) image sensors. Nowadays, as imaging lenses for forming a subject image on those image sensors, lenses made of resin are used that can be mass-produced inexpensively and that thus contribute to cost reduction.
As such imaging lenses, in particular as imaging lenses used in imaging devices (so-called camera modules) incorporated in portable terminals, there are widely known those comprising three plastic lenses and those comprising one glass lens and two plastic lenses. Inconveniently, however, these imaging lenses do not promise easy compatibility between further size reduction and higher mass-producibility due to technical limitations.
As one measure to overcome this inconvenience, Patent Document 1 listed below deals with the replica method. The replica method is a method for forming a large number of lenses (lens elements) on a single lens flat plate (wafer). And Patent Document 1 discloses an imaging lens comprising a cemented compound lens (lens block) formed by the replica method. Incidentally, in a cemented compound lens in an imaging lens, a diffractive surface and a refractive surface are formed simultaneously on a lens flat plate, so that with those surfaces the imaging lens corrects chromatic aberration.
On the other hand, Patent Document 2 listed below discloses a cemented compound lens comprising a lens flat plate, an object-side lens with respect to the lens flat plate, and an image-side lens with respect to the lens flat plate. In this cemented compound lens, from the viewpoint of aberration correction, the lens flat plate, the object-side lens, and the image-side lens are given indices of refraction, and also Abbe numbers, that do not differ much from one another.    Patent Document 1: JP-A-2006-323365    Patent Document 2: JP-B-3929479